1. Field of the Invention
The present invention relates to a control method of and system for a lock-up type automatic transmission for an automotive vehicle.
2. Description of the Prior Art
An engine provided with fuel cut means has been developed wherein the fuel cut means suspends the supply of fuel to the engine (hereinafter called as a fuel cut) after the accelerator pedal has been released to cause the vehicle to begin a coasting operation and the suspension of the supply of fuel to the engine continues as long as the engine is in an operation state which allows the engine to resume combustion upon the resumption of the supply of fuel to the engine. The predetermined state in which the engine can resume combustion may be defined by the engine speed or the vehicle speed and may be identified by a state when the engine speed is higher than a predetermined engine speed value or a state when the vehicle speed is higher than a predetermined vehicle speed value. The supply of fuel to the engine is resumed (the resumption of the supply of fuel) when the engine speed has dropped below the predetermined engine speed value while the vehicle is coasting.
With an automotive vehicle which is installed with an engine provided with such fuel cut means and which is capable of running by transmitting the power from the engine to traction wheels via an automatic transmission, since a torque converter of the automatic transmission cannot establish a non-slip direct drive connection between the engine and the traction wheels, the magnitude of a driven power, viz., a rotational power transmitted from the reaction wheels to the engine when the vehicle is coasting or decelerating, is relatively small. Thus, a drop in the engine speed upon releasing an accelerator pedal is more rapid and greater than that in the case of an automtive vehicle installed with a manual transmission. In the excessive case, the engine tends to stall during such a rapid braking operation as to bring the vehicle to a stop because the actual supply of fuel to the engine after the resumption of the supply of fuel does not occur at an appropriate timing with the drop of the engine speed and is apt to be delayed. In order to reduce this tendency, it has been the common practice to set the engine speed value at which the supply of fuel is to be resumed at a sufficiently high value in the case of the automotive vehicle having the automatic transmission as compared to the case with the automotive vehicle having the manual transmission. Thus, the time period when the engine speed remains above the predetermined engine speed value is short.